Explosive charge for delay fuze



Dec. 23, 1958 TIME (MICROSECONDS) H. P JENKINS ET AL 2,865,726 EXPLOSIVECHARGE FOR DELAY FUZE Filed June 3, 1952 A 0 2o 40 so so 95 I00CONCENTRATION (WEIGHT LEAD Azms) INVENTORS CHARLE S H. SHONATE HUGH P.JENKINS, JR

ATTO NEYS tat 2,865,725 Patented Dec. 23, 1953 EXPLOSIVE CHARGE F ORDELAY FUZE Hugh P. Jenkins, Jr., and Charles H. Shomate, China Lake,Calif.

Application June 3, 1952, Serial N 0. 291,586 3 Claims. (Cl. 52-2)(Granted under Title 35, U. S. Code (1952), see. 266) The inventiondescribed herein may be manufactured and used by or for the Governmentof the United States of America for governmental purposes without thepayment of any royalties thereon or therefor.

This invention relates to a method for controlling the rate ofdetonation of explosives; to new explosive compositions, and articlesmade therefrom.

There are many applications of explosives wherein control of theirdetonation rate is highly advantageous. This is particularly true in theuse of military explosive missiles where penetration of the targetbefore explosion is often necessary in order to realize the maximumeffect. The ordinary primary explosives such as lead azide when usedalone have too short a delay for use in fuzes to give maximum etfectagainst certain targets which are frangible, such as airplane wings.

In the past, delay in fuzes has usually been obtained by such devices aspyrotechnic trains, bafiie elements, me chanical devices and others.These prior devices have many disadvantages for this use, including thefact that they do not give reproducible delays and, generally, shortdelays cannot be obtained with them. In addition, the mechanical deviceshave restricted application because of space requirements. In general,delays have not been obtained in fuzes for military explosive missilesby modification of the composition of the initiating explosive.

it is an object of this invention to provide a method for controllingthe rate of detonation of initiating explosives.

The above and other objects are accomplished by adding to the primaryexplosive a waxy material in amounts determined by the rate ofdetonation required.

The waxy materials include fatty acids such as stearic acid, metallicsalts of fatty acids such as aluminum stearate and many others, theessential requirement being that the material have waxy properties sothat the particles of explosives are coated and the interstices betweenthe particles are filled up. The invention has direct applicability toprimary explosives such as lead azide and mercury fulminatc, but couldconceivably be used to modify the detonation rate of other types ofexplosives.

The invention is illustrated by reference to the tabulated results setforth in Tables I and IT below and the accompanying drawings hereby madea part of this specification in which,

Fig. 1 is a vertical section of the apparatus used to test the explosivemixtures, and

Fig. 2 is a graph of the results given in Table I in which delay time isplotted against percent lead azide in the mixtures.

Various mixtures of lead azide and aluminum stearate and lead azide andstearic acid were tested by measuring the time elapsing betweeninitiation and detonation of a small column of each mixture.

Referring to Fig. 1, the numeral designates an outer wall of strongmaterial such as brass or stainless steel having a cylindrical chamber11 therein. The exit of the chamber is partially closed with a metalplug 12 2 which is crimped in place as shown. The initiating device 13such as a graphite igniter bridge is placed at the opposite end of thechamber, and is provided with an ignition device 14 which may be of theelectrical or other type. The lead azide mixture 15 was placed insidethe chamber 11 in contact with the graphite igniter bridge. The lengthof the column, as shown in the tables, was about .9" long. The describedtest device could be used as a delay element for a fuze. The column ofexplosive was initiated by a condenser discharge which also started twotiming devices, each adapted to record the time interval betweeninitiation of the explosive, and the arrival of a detonation wave at theexit of the column. Two RCA type WF99B counters, connected in parallel,were set to start on one electrical pulse and to stop on the second. Amake-type switch was used at the exit of the column. It consisted of twoleaves of 0.004-inch copper foil, separated by a 0.004-inch-thick paperwasher fastened together with a single Wrapping of scotch tape. Thisswitch was then clamped down on top of the bomb with the vent hole inline with the center of the paper washer. The blast from the venteffected a contact between the two copper strips, completing thestop-circuit and sending a pulse to stop the counters. This arrangement,when tested, always gave a positive, reproducible operation. The timingcircuit is not shown as it is considered standard apparatus. Theinterval between two pulses was read directly from the indicatingdecades of the counters. The accuracy claimed for the counters is onemillionth of a second. The two counters were used as checks against eachother and when one of the counters failed to start or stop, or thereadings disagreed by more than 5 microseconds, the readings werediscarded.

It should be noted that the instruments measure the lapse of timebetween a sending of a pulse to fire the explosive and a receiving of areturn signal. In addition to the time required to detonate the columnof lead azide after initiation, this time interval includes, of course,the times required to send a pulse to the igniter bridge, to explode thegraphite film, to initiate the lead azide, to send the shock wavethrough the brass plug, to close the make-switch, and to send a stoppulse back to the counters. In order to ascertain the magnitude of theseadditional time intervals, at special bomb with an explosive column onlyinch long was fired. The time for the detonation to travel through theinch column was considered negligible. The total time lapse, as read onthe counters, was only 16 microseconds. This indicated that the timingsystem was satisfactory in that this time interval was small comparedwith the recorded times of several hundred microseconds usuallyobtained.

Standard military lead azide was used in all the compositions. Usuallyone gram of azide was weighed accurately and mixed thoroughly with acalculated amount of the diluent. Mixing was accomplished by a smalltumble mill designed for the purpose. The azide mixtures were loadedinto the bombs by increments, the increment being compressed on top ofthe igniter bridge by a polished As-inch steel plunger, the secondincrement on top of the first, and so on, until the column was filledunder 12 tons per square inch pressure. A greater degree of homogeneityin the column was obtained by this method of loading. In general, thecompressed material was a hard mass, which did not lose'its form whenthe pressure was removed. The aluminum stearate powder used was a BraunCorporation product. The stearic acid was a reagent-grade materialpurified by distillation.

The results obtained on a number of compositions are given in thefollowing tables.

TABLE I Lead azide-aluminum stearate mixtures Mixture" I ength Time 7Average Stand ard Composition of Chargetmicrosec.) (microseel) Deviation(wt. percent azide) (int) (microsee;)

Y 0 923 173 0.891 461 (i8 4 14s 57.14, 0. 910 83 108 35 TABLE II Leadazzde-stearzc aczd mixture Mixture i Weight Length Time AverageComposition of Charge of Charge (microsec.) (mici-osec.) (wt. percentazide) (g.) (in.)

0. 292 0. 890 no fire 0.324 0015- i 652 i 591 0.317 0.808 530 It willbeInoted from the tables that'the optimum compositionof lead'azidc andaluminum stearate is one containing about144.44 percent lead azidewherein an average-delay of 816 micro-seconds is obtained with astandard deviation of 108 microseconds-between the tests. With a 50;percent. mixture of lead azide, an average delay 06354., microseconds isobtainedWith a standard deviation of l6l'microseconds. With acomposition containing 57.14 percent azide, an average delay of 108microsecondsis obtained with a standard deviation of 35 microseconds.The standard deviation is lower than any heretofore obtained for thisshort delay and is quite small for this type of statistical phenomenon.

Referringlto Fig. 2, it will be noted from the graph that, as would beexpected with nearly pure lead'azide, extremely short delaysare obtainedThis delay does not increase appreciably until'the percentage of leadazide decrease to-about60. At somewhere between 60' and 40 percent-leadazide, there is a'sharpdeviation inthe delay time and delays of 800seconds and above'are reached-within this percentage range. It will benoted that ifthe results for stearic acid were-also plotted on the'graph, the critical range wouldalso fall within the 40' and 60 percentlead azide range, as;th emixture did notfire'at 44e44 percent lead azide"and two firings of a 50 percent-lead azide mixture gave an averagedelay of-59l seconds.

Although the cause of this unexpected phenomenon is not definitelyknown, it is believed to be due to a aseeyrae 1,488,787 Harle Apr. 1,1924 7 2,146,033 Seavey et al. Feb. 7, 1939 2,402,235 Burrows et a1.June 18, 1946 d changeover from detonation to defiagration at somewhereabove 60 percent of lead azide; This would seem to be substantiated bythe fact that, as is well known, lead azide can be dead pressed to thepoint where it cannot be detonated but can be deflagrated. This resulthas been explained by the fact that loose explosives are most readilydetonated because of the possibility of formation of pressure gradientsin the interior ofthe mass. Applying this theory to the instantinvention, it is believed that the use of the waxy materials fills theinterstices between the particles, thus preventing the formation ofpressure gradients so-that deflagration and not detonation occurs withthe result that delays are obtained.

As is illustrated by the above results, it is an advantage of theinvention that the rate of detonation of a primary explosive such aslead azide can be reproducibly con trolled, depending upon the desireddelay, by the addition of the type diluents disclosed herein. This is afar more reliable method ofobtaining reproducibility than that ofcompressing the explosive and the use of the other prior methodsmentioned above with their attendant disadvantages. Another advantage isthe fact that extremely short controlled delays can be obtained withthis invention; A further advantage of the invention is that its usereduces space requirements for fuzes, as the space for the, delay-trainis dictated onlyby the amount of explosive mixture used.

Obviously, many modifications and variations of the teachings. It is,therefore, to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically describedWhat is claimed is:

1. An explosive charge for use as a short time. delay fuze consistingessentially of a compressed mass of homogeneously mixed particles oflead azideand a material chosen from the class consisting of aluminumstearate and stearic acid, the particlesof'said material being presentinan amount from about40% to about 60% by weight. v

2. An explosive charge for use as a short time delay fuze consistingessentially of a compressed mass of homogeneously mixed particles oflead azide and aluminum stearate, the aluminum-stearate particles beingpresent in an amount from about,40% to about 60% by weight.

3. An explosive charge for use as aishort timedelay fuze consistingessentially, of, ancompressed. mass of homogeneously mixed particles oflead azide and .stearic acid, the stearic acid particles being; presentin an amount from about.40% to about 60% byweight.

ReterencesCited in thefile of this .patent UNITED'STATES PATENTS

1. AN EXPLOSIVE CHARGE FOR USE AS A SHORT TIME DELY FUZE CONSISTINGESSENTIALLY OF A COMPRESSED MASS OF HOMOGENEOUSLY MIXED PARTICLES OFLEAD AZIDE AND A MATERIAL CHOSEN FROM THE CLASS CONSISTING OF ALUMINUMSTEARATE AND STEARIC ACID, THE PARTICLES OF SAID MATERIAL BEING PRESENTIN AN AMOUNT FROM ABOUT 40* TO ABOUT 60% BY WEIGHT.